The reduction of nitrogen oxide (NOx), e.g., nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), in exhaust gas is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry. One approach uses three-way conversion catalysts to treat the exhaust gases of spark-ignited gasoline internal combustion engines for the reduction of NOx emission, since the exhaust contains minimal oxygen.
However, fuel economy and global carbon dioxide (CO2) emission concerns have made it desirable to operate engines under lean-burn conditions to realize a benefit in fuel economy. Under such conditions, the exhaust streams contain excessive oxygen, and accordingly, conventional three-way conversion catalysts are generally inefficient in the reduction of nitrogen oxides. For example, a lean burn engine exhaust stream has from about 1% to 10% excess oxygen relative to the amount of fuel. Further, exhaust streams of compression-ignition engines such as diesel engines also have high oxygen content. For example, the exhaust stream may contain from perhaps about 2-20 molar % oxygen based on the total exhaust, and, in addition, contains a significant amount of particulate emissions.
Hydrocarbon (HC), particulate, or NOx emissions from an exhaust or effluent stream may be treated with a non-thermal plasma reaction in conjunction with a NOx catalyst, adsorber, or storage compound. Typically, the various NOx compounds are converted to NO2, which, in the presence of HCs and water, is converted into nitrogen gas (N2), oxygen gas (O2), carbon dioxide (CO2).
In addition to the NOx, the exhaust gases typically contain sulfur compounds such as sulfur dioxide (SO2) gas and particulate sulfate (SO4), which are produced by the burning of sulfur contained in the fuel. The NOx catalyst oxidizes SO2 to sulfur trioxide (SO3) in oxygen-rich atmospheres. Still further, SO3 reacts readily with water vapor, which is also contained in the exhaust gases, to produce sulfite ions and sulfate ions. The sulfite ions and sulfate ions react with the NOx catalyst to produce sulfites and sulfates. The resulting sulfites and sulfates adversely affect the NOx storage reaction. Thus, such sulfites and sulfates are commonly referred to as NOx poisons.
Therefore, as is apparent from the above brief description of NOx removal, such NOx poisons result in degraded exhaust purification. What is needed in the art is a sulfur resistant NOx catalyst.